To reduce the chance of recurrent hyperthyroidism, two methods of subtotal thyroidectomy were performed and compared. From January 1998 to December 2002, 340 patients were operated on with subtotal thyroidectomy. They were prospectively randomized into two groups. Group A included 166 patients and group B 174 patients. Group A patients had a 2.5 x 1 x 1 cm thyroid remnant on each side and group B patients had a 2.5 x 1 x 1 cm remnant on one side plus total lobectomy on the other side. Thyroid function tests including T(3), T(4), TSH, and antimicrosomal antibody (AMA) were checked preoperatively and in the follow-up period of 3 months, and later up to 26.4 +/- 1.1 months (mean +/- SE). The age, sex, duration of oral medicine, and blood loss of the two groups were not significantly different during surgery and the follow-up period. The operative time was less in group A (113 +/- 3.3 minutes) than that in group B (131 +/- 3.2 minutes) (p < 0.001). In the long-term follow-up period, recurrent hyperthyroidism was noted in 15 patients in group A and 3 patients in group B. The difference was significant (p = 0.003). Hypothyroidism was noted in 35 of the group A patients and in 46 of the group B patients. The differences between the two groups regarding hypothyroidism was not significant (p = 0.181). Multivariate logistic regression analysis revealed preoperative titers of AMA > or = 6400, which was the only factor affecting the incidence of hypothyroidism in the later follow-up period. In consideration of hypothyroidism, recurrent hyperthyroidism, and postoperative complications, subtotal thyroidectomy with total lobectomy plus subtotal lobectomy provides a better outcome than bilateral subtotal lobectomy.